Hitherto, there has been known a technique for controlling an actuator device provided in an actual vehicle, whereby a model representing the dynamic characteristics of a vehicle is used to set the target values of the motional state amounts, such as a yaw rate and a lateral velocity, of a vehicle, and then an actual device provided in an actual vehicle is controlled such that the state amounts of the actual vehicle follow the target values, as disclosed in, for example, Japanese Patent Application Laid-Open No. H11-91608 (hereinafter referred to as patent document 1) and Japanese Patent Application Laid-Open No. 2000-15938 (hereinafter referred to as patent document 2).
According to the technique disclosed in these patent documents 1 and 2, a target yaw rate and a target lateral velocity are set from a detection value of a vehicle velocity and a detection value of a steering angle of a steering wheel by a dynamic characteristic model having a yaw rate and a lateral velocity of a vehicle as state amounts. Then, a target value of a steering angle of a rear wheel of the vehicle required relative to the target yaw rate (a first target rear wheel steering angle) and a target value of a steering angle of a rear wheel of the vehicle required relative to the target lateral velocity (a second target rear wheel steering angle) are calculated. Further, a value (a weighted mean value) obtained by linearly coupling the first target rear wheel steering angle and the second target rear wheel steering angle is determined as the final target value of the steering angle of the rear wheel. Then, an actuator for steering the rear wheel is controlled to make the steering angle of the rear wheel of an actual vehicle follow the target value.
Further, as disclosed in, for example, PCT international publication WO2006/013922A1 (hereinafter referred to as patent document 3), the present applicant has proposed a technique whereby an actuator device of an actual vehicle is controlled to bring a state amount of the actual vehicle close to a state amount on a dynamic characteristic model of the vehicle and also the dynamic characteristic model is operated (an additional control input is supplied to the dynamic characteristic model) to bring a state amount on the dynamic characteristic model close to a state amount of the actual vehicle.
Meanwhile, when carrying out the control to make a state amount related to a motion of an actual vehicle follow a state amount on a dynamic characteristic model of a vehicle, if a change in an input, such as a steering angle of a steering wheel, causes the state amount on the dynamic characteristic model of the vehicle to develop an oscillatory response, then a state amount of the actual vehicle, which is to follow the state amount on the dynamic characteristic model, will also develop an oscillatory response. It is considered desirable, therefore, that the response characteristic (the transient response characteristic) of a state amount on the dynamic characteristic model relative to a change in an input, such as the steering angle of the steering wheel, exhibits a response characteristic having a highest possible attenuation property independently of a motional state of the actual vehicle in order to maintain a good behavior characteristic of the actual vehicle as much as possible.
Here, in the present description, “the high attenuation property” means a short time constant of attenuation of an amplitude value of an oscillatory component of a state amount (a control amount) when an input to a target system is changed in steps, that is, a high attenuation speed of the amplitude value of the oscillatory component. Incidentally, the so-called critical braking (critical damping) or over-braking (over-damping) response characteristic is a non-oscillatory response characteristic with no oscillatory component; however, it is regarded as a characteristic with a highest attenuation property.
Nevertheless, it has been difficult for the technique disclosed in the aforesaid patent documents 1 and 2 to fully satisfy the above requirement for the following reason. In general, the response characteristic of a state amount, such as a yaw rate or a lateral velocity, of an actual vehicle relative to a change in the steering angle of a steering wheel tends to exhibit an oscillatory response characteristic (a characteristic in which a state amount converges to a steady-state value while oscillating) when a traveling velocity is relatively high.
Hence, if the response characteristic of a dynamic characteristic model of a vehicle is set to a characteristic with a high attenuation property, such as a critical braking or over-braking characteristic (non-oscillatory characteristic), independently of the traveling velocity of the actual vehicle (at an arbitrary traveling velocity) according to the technique disclosed in patent documents 1 and 2, then there will be a significant discrepancy between the response characteristic of a state amount of a vehicle on the dynamic characteristic model and the response characteristic of a state amount of an actual vehicle especially when the traveling velocity of the vehicle is high. As a result, the difference between a yaw rate and a lateral velocity as the state amounts on the dynamic characteristic model and a yaw rate and a lateral velocity of an actual vehicle according to the technique disclosed in patent documents 1 and 2 will become large. Consequently, a required manipulated variable of an actuator device for making a state amount of the actual vehicle follow a state amount on the dynamic characteristic model tends to be excessive. As a result, a situation wherein it actually becomes impossible to satisfy the required manipulated variable within the capability of the actuator device frequently occurs. Further, in this case, it actually becomes difficult to make the state amount of the actual vehicle follow the state amount on the dynamic characteristic model.
Hence, according to the technique disclosed in patent documents 1 and 2, it is actually impossible to set the response characteristic of a state amount on the dynamic characteristic model of a vehicle to a characteristic with a high attenuation property, such as a non-oscillatory characteristic, independently of a traveling velocity of an actual vehicle. This means that, actually, the response characteristic of a state amount on the dynamic characteristic model of a vehicle can be set to a characteristic with a high attenuation property only in limited motional states (motional states wherein the response characteristic of a state amount of the actual vehicle becomes a characteristic with a high attenuation property), such as in a situation wherein the traveling velocity of a vehicle is low. For this reason, the technique disclosed in patent documents 1 and 2 cannot fully satisfy the aforesaid requirement.
Meanwhile, according to the technique disclosed in the aforesaid patent document 3, in addition to operating the actuator device of the actual vehicle on the basis of the difference between a state amount of the actual vehicle and a state amount on the dynamic characteristic model, the dynamic characteristic model of the vehicle is also operated so as to bring the state amount on the dynamic characteristic model close to the state amount of the actual vehicle. In other words, the difference is fed back not only to the actual vehicle but also to the dynamic characteristic model. This makes it possible to restrain the difference between the state amount of the actual vehicle and the state amount on the dynamic characteristic model from becoming excessive. Thus, as shown in patent document 3, if the difference between the state amount of the actual vehicle and the state amount on the dynamic characteristic model is fed back to both the actual vehicle and the dynamic characteristic model, then it is considered possible to restrain a required manipulated variable of the actuator device from becoming excessive even when there is a relatively large discrepancy between the response characteristic of a state amount of the actual vehicle and the response characteristic of a state amount of the vehicle on the dynamic characteristic model. Therefore, it is considered possible to make the state amount of the actual vehicle properly follow the state amount on the dynamic characteristic model.
Hence, as disclosed in patent document 3, if the difference between a state amount of the actual vehicle and a state amount on the dynamic characteristic model is fed back to both the actual vehicle and the dynamic characteristic model, then it is considered possible to set the response characteristic of a state amount on the dynamic characteristic model of a vehicle to a response characteristic with a high attenuation property independently of a motional state of the actual vehicle.
The present invention has been made in view of the background described above, and it is an object of the present invention to provide a vehicle control device capable of properly controlling an actuator device of a vehicle, independently of a motional state of the vehicle, by using a dynamic characteristic model with a high attenuation property such that the state amount related to a motion of an actual vehicle approximates the state amount of a vehicle on the dynamic characteristic model, thus allowing a good behavior characteristic of the vehicle to be maintained.